Electrocardiograph telemetry system having circuitry for indicating inoperative conditions

ABSTRACT

Special circuitry in the transmitter of an electrocardiograph (ECG) telemetry system detects various malfunctions of the system and changes the transmitted signal to indicate their presence to the system receiver. When the transmitter detects that an input electrode has become detached from the patient, it changes the frequency of the subcarrier signal to indicate this problem to the receiver. The receiver monitors the subcarrier frequency and flashes an alarm light when the frequency corresponds to the electrode inoperative condition. When the voltage output from an aging battery becomes too low to adequately energize the transmitter, special circuitry stops the transmission of signals from the transmitter. When the receiver cannot detect a transmitted signal, it indicates that either the battery needs replacement or the transmitter is out of range.

United States Patent Eillman et a1.

1451 Oct. 23, 1973 [75] Inventors: Richard F. Dillman, Lexington; primary Examiner Albert Mayer James L. Larsen, Needham Heights; A0mey A Smith Alfred M. Nardizzi, Dedham, all of Mass 57 ABSTRACT Assigncel Hewlett-Packard p y PaIO Special circuitry in the transmitter of an electrocardio- AIIO CaIIfgraph (ECG) telemetry system detects various mal- [22] Filed: 14 1971 functions of the system and changes the transmitted signal to indicate their presence to the system rel l PP 207,859 ceiver. When the transmitter detects that an input electrode has become detached from the patient, it 52 us. c1 325/48, 128/206 R, 12s/2.1 A, change? the frequency Ofthe W sighal i- 325/344, 325/346, 325/348, 325/349 cate th1s problem to the rece1ver. The receiver moni- [51] Int; Cl. I-I04b 1/00 tors III? sII-bcarrIer frequency and flashes an alarm [58] Field of Search 128/2R, 2.06 R.2.1 A; light when the frequ'ehcy P the electrode 325/48 45 344 348, 349, 478 346, 417, inoperative condition. When the voltage output from I I I 4184120, 423 an aging battery becomes too low to adequately ener- 1 I gize the transmitter, special circuitry stops the trans- [56] References Cited mission of signals from the transmitter. When the receiver cannot detect a transmitted signal, it indicates 3 603 88 :I PATENTS 28/2 1 A that either the battery needs replacement or the trans- OI'I'IIOII I It t f 3,524,058 8/1970 Robertson 128/2 R m1 er S CU 0 range 3,681,698 8/1972 McEvoy 325/478 7 Claims, 15 Drawing Figures F w w l 3 1 Y2 l6, 0, 18 r T DEFIBRILLATION VOLTAGE I INPIUTSQ PROTECTION AMP CONTROLLED FILTER I l MULTIVIBRATOR i I l 19 I INPUT l I INOPERATIVE I J l I r I OUTPUT AMP AND x4 VOLTAGE I HLTER FREQUENCY CONTROLLED I I MULTIVIBRATOR CRYSTAL I E OSCILLATOR I I :17 I l i REGULATOR OSCILLATOR BIAS I I AND SHUTDOWN 1 1 I L 6 11 1 l PAIENIEUUCT 23 ms 3,768,017

sum 020F12 FIG. 2c FIG. 2b FIG. 2a

FIG. 2f I FIG. 22 FIG. 2d

FIG. 2b FIG. 2g

IFIG- 2 PAIENIEflncr 23 ms SHEET 12 0F 12 NOISE INPUT I IF BANDWI DTH,

DISCRIMINATOR OUTPUT VOLTAGE 9 Fiure 7 ELECTROCARDIOGRAPH TELEMETRY SYSTEM HAVING CIRCUITRY FOR INDICATING INOPERATIVE CONDITIONS BACKGROUND OF THE INVENTION A patient recovering from heart surgery or suffering a myocardial infarction must be kept under constant observation until his heart condition improves. Monitoring the electrocardiac signals, sometimes called ECG signals, produced by the expansions and contractions of the patients heart is a common method of observation during this time. These electrocardiac signals are present on the skin and throughout the body. They are a valuable medical indicator because their shape and repetition rate can indicate to a trained observer whether the heart is operating properly or nearing a dangerous condition.

During the initial phase of a heart patients recovery, he is bedridden and directly connected to a bedside monitor, such as an electrocardiograph. The monitor is usually wired to electrodes that are attached to the skin near the heart. The electrodes detect the electrocardiac signals that are circulating on the skin, and the wires transmit them to the monitor.

When a patients condition improves, it is often desirable to let him move about. This is difficult if he remains connected to the bedside monitor because the wires restrict his movement. To remedy this problem, a telemetry system is sometimes used to replace the direct wired connection.

The telemetry system includes a portable transmitter carried by the ambulatory patient and a stationary receiver connected to themonitor. Electrodes still sense the electrocardiac signals, but now the signals are transmitted by radio waves to the receiver. At the receiver, the transmitted signal is demodulated and the resultant electrocardiac signal is conveyed to the monitor. With such a telemetry system, a heart patient can move about while his electrocardiac signals are kept under constant surveillance.

If a heart monitoring system becomes inoperative, a special indication should be given to the monitor operator so that the fault'can be quickly corrected and so that the inoperative condition will not resultin-confusion and create a false heart rate alarm. Because of the inoperative condition is detected to prevent the output of erroneous ECG signals to the monitor.

This invention monitors the input signal to the transmitter to detect when an electrode detaches from the patients skin or when the input wires develop an open circuit. When such,a condition is detected, the subcarrier frequency of the transmitter signal is changed by special circuitry in the transmitter. The receiver is designed to detect this frequency change and light an alarm light to signal the problem to theoperator;

This invention also monitors the voltage output of the transmitter battery. When the batteryvoltage begins to inhibit'regulation of the power supply, signal transmission from the transmitter is prevented. The receiver examines the received signal for noise or interference conditions. When it detects an input signal containing only noise or interference and not the ECG signal, it lights an alarm light to indicate that the transmitter is either inoperative or out of range.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing the preferred embodiment of the inoperative circuitry in a conventional transmitter.

FIG. 2. and 2(a-h) are block and schematic circuit diagrams showing the circuit configuration of the transmitter including the input circuitry that detects an open circuit at the input terminals. 7

FIG. 3 is a block diagram'sh'owing the preferredembodiment of the inoperative circuitry in the receiver.

FIG. 4 is a graphical diagram showing the transfer characteristics of a frequency discriminator.

FIG. 5 is a graphical diagram showing the noise output voltage of the frequency discriminator.

' FIG. 6 is a'graphical diagram showing the operation of comparators-on the output of a frequency discriminator.

increased movement of'an-ambulatory patient, there 7 are more problems involved 'with a telemetry monitoring system than with a stationary moniton'Patient movement may disconnect an electrode, stopping detection of the electrocardiac signals, or it may shift'the position of anelectrode, weakening the detected ECG signals. An ambulatory patient may also move out of the range of the receiverand ruin the reception.-

Since the transmitter must be portable, it usually contains a battery for a .power source. Whenthe voltage output of the battery decreases with age, the power supply may become unregulated and cause the transmitter circuitry to drift with the unregulated supply voltage. This will cause erroneous information to be transmitted to the receiver.

SUMMARY OF THE INVENTION The present invention detects various inoperative conditions that are common to ECG telemetry systems, and it indicates to the system operator that these conditions exist. It also inhibits the receiver output when an FIG. 7 is a graphical diagram showing the charging characteristics of the detector circuits.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2, two input terminals 1 are connected'to sensing electrodes'that are attached to a patient to pick up his ECG signals; The input terminals are connected to circuitry 2 that protects the transmitter from damage by the high voltages used in defibrillation. After passing through an amplifier, theECG signals are used to frequency modulate a subcarrier signal generated by the voltage controlled multivibrator 3. After the'modulated subcarrier is filtered, it is used to frequency modulate a carrier signal generated by the voltagecontrolled crystal oscillator '5. The resulting F M-FM signal is multiplied in frequency by a factor of four, is filtered, and is then transmitted from the antenna 10. A power supply 7 and aregulator 9 provide the energy to operate the transmitter. Since the transmitter must be portable, the power supply is usually a battery.

If an input electrode becomes detached from the patient,.or if one of theinputs develops an open circuit,

- the input inoperative circuitry 12 detects the condition and reduces the subcarrier frequency to signal this inoperative condition to the receiver. The circuitry detects the open condition by sensing the unusually low input current associated with an open circuit.

Referring now to the schematic diagram of FIG. 2 which shows the input inoperative circuitry, a detached electrode causes the signal at one of the inputs 1 to float towards the voltage level of the supply 20. This will shut off the corresponding transistor of the transistor pair 16. This, in turn, will saturate the corresponding transistor of the transistor pair 17. The saturated transistor will draw an increased current through resistor 18 and increase the emitter-base voltage of transistor 19. This will turn on transistor 19 which is normally off. The collector of transistor 19 is connected to a voltage divider that controls the output frequency of the voltage controlled multivibrator.

The output frequency of a voltage controlled multivibrator, a device well known in the art, is dependent on the input voltage. Transistor l9 and its output voltage divider are constructed to decrease the frequency of the multivibrator to approximately one half or less of its normal frequency. Since this is the subcarrier signal, the input inoperative signal is transmitted to the receiver by the decrease in the subcarrier frequency. Circuitry in the receiver will detect this frequency change and indicate the input inoperative condition.

The carrier oscillator includes a single bipolar transistor Q connected in a common base configuration. The collector of transistor O2; is tuned by the resonant circuit of capacitor C and inductor L to maximize the power gain of the stage at the desired frequency. Positive feedback to sustain oscillations is provided by the capacitive divider. formed of capacitors C and C and by the feedback path including varactor diode CR inductor L capacitor C and crystal resonator Y connected to the emitter of transistor Q Capacitor C is a signal bypass and the frequency of oscillation is determined primarily by the crystal Y Experimental tests indicate that spurious oscillations are generated by the saturation of the collector-base junction of Q21 under normal operating conditions. This junction of the transistor is shunted by. a metal semiconductor Schottky-barrier type diode CR that has lower saturation voltage than that of the collectorbase junction to prevent saturation with concomitant generation of spurious frequencies. This simplifies the transmitter circuitry by reducing the filtering requirements and also greatly facilitates the tuning-up procedures required to establish proper operation on the assigned frequency.

Referring again to FIGS 1 and 2, the oscillator bias and shut down circuitry 1 1 detects a low battery supply 7. A weak battery causes the regulator 9 to become ineffective, and signals generated during this condition may be erroneous because of supply voltage drift. The regulator includes a series-pass transistor, a device well known in the art. As the battery output current decreases, the voltage drop across the emitter-collector terminals of the series-pass transistor decreases. This causes the transistor to begin to saturate and draw more base current. When the transistor saturates, the regulator loses control over the output of the voltage supply.

To detect this problem, the oscillator shut down circuitry 11 monitors the base current in the series-pass transistor. When the current exceeds a given value, the shut down circuitry prevents the generation of the carrier signal and stops the radiation from the transmitter. In the process of the transistor 6 turning on to prevent the oscillator from operating, it draws more current than is normally supplied to the oscillator, thus further reducing battery voltage and assuring that the oscillator remains locked off. Without this current drain to replace the oscillator current drain, the reduced current drain on the battery would result in increased battery voltage sufficient to reactivate the oscillator. This would produce an unstable condition that would result in intermittent transmission. The present circuit thus assures that the transmission of erroneous signals due to an unregulated power supply is prevented.

Referring now to FIG. 3, wherein is shown a functional diagram of the receiver circuitry, the FM-FM signal transmitted from the ambulatory patient is received at the antenna 25 of the stationary receiver. After amplification, this signal is demodulated to an IF signal in the conventional manner. It is frequency mixed at the mixer 27 with a local oscillator signal which is generated by the local oscillator 30. After passing through the discriminator, the resultant signal at node 33 is the frequency modulated subcarrier signal carrying the ECG information.

After passing through a buffer amplifier and a filter, the signal is converted from a sinusoid to a square wave by the subcarrier amplifier 35. The square wave then drives a monostable multivibrator 36 that gives a pulse output for every positive or negative transition of the square wave. The multivibrator output carries the ECG signal in its frequency modulated pulse train. The pulse train is time averaged and filtered by the ECG filter 38, and the filter output is the original ECG signal detected by the electrodes attached to the patient. This signal is amplified by the output amplifier 39. The output terminal 40. can be connected to an electrocardiograph or any other appropriate monitoring device. Thus, the receiver performs two demodulations of the FM-FM input signal to extract the original ECG signal.

The receiver detects a detached electrode by monitoring the frequency of the subcarrier signal. Since the pulse output from the multivibrator 36 is directly proportional to the ECG modulated subcarrier, the receiver compares the period between the pulse to a pre determined period. This is done by the period comparator 42 that is connected to an output from the multivibrator. The pulse output is used to discharge a capacitor. When there is no pulse, the capacitor charges. Consequently, for lower frequencies, i.e., longer periods between pulses, the capacitor charges to higher voltages. For a low enough frequency, the capacitor charges to a voltage high enough to trigger the electrode inoperative circuitry.

The period comparator is adjusted to trigger the turn off delay 44 when the pulse train frequency corresponds to a subcarrier frequency indicative of the electrode inoperative condition. For the transmitter shown in FIG. 1, the comparator would be set to trigger when the subcarrier is at one-half its normal frequency which is outside the normal operating band of frequencies. The turn-off delay energizes the electrode inoperative indicator 50 to signal to the operator that this problem exists. The turn-off delay also shuts down the output from the receiver by energizing the output hold off circuitry 41. This is done to prevent erroneous output signals.

The receiver senses the signal conditions that indicate when the patient is out of range or when the transmitter is inoperative. This sensing circuitry includes two peak-to-peak detectors 52, 54 connected to the output of the buffer amplifier 34 and a window comparator 56 that examines the output from the detectors. The comparator drives circuitry 60, 61 that controls the local oscillator frequency. It also controls the output hold off circuitry 41 and the range/battery inoperative indicator 48. When the window comparator detects an inoperative condition, it disables the electrode inoperative circuitry 42, 44 to prevent erroneous indications of detached electrodes.

The input to the peak-to-peak detectors 52, 54 is the ECG modulated subcarrier signal. These detectors, well known in the art, convert the peak-to-peak voltage of the FM subcarrier signal to a representative DC. voltage. One detector 52 holds the peak-to-peak voltage for a relatively long time while the other detector 54 holds the voltage for a much shorter time. Each detector includes two capacitors that charge respectively to the peak voltage of the negative half cycle and the peak-to-peak voltage excursion. The period of measurement of a detector is determined by the discharge times of the capacitors.

The range/battery inoperative detection may be considered as working on the amplitude of the demodulated subcarrier. The output of an FM discriminator is a wave having an amplitude that is related to the frequency deviation of the carrier, as shown in FIG. 4. The discriminator output for a noise input signal thus typically has a higher peak amplitudecand, for interference input signals, may generally have either higher'or lower peak amplitude than on applied input signals, as shown in FIG. 5. As an example, consider an AM signal as an interference signal applied to the frequency discriminator. Since the carrier frequency does not deviate with time, the discriminator outputmay be simply a static value that can be readily analyzed.

By using the comparators 56 and 56', it is possible to set a narrow window about the discriminator output voltage and require that the peak output amplitude remain in the window selectively, as shown in FIG. 6, to unlock the inoperative circuits. The time constants of the detectors 52, 54 may be chosen such that for transitions between noise and interference, there is no interim period where the inoperative circuits unlock, i.e., one comparatorwould be activated before the other comparator releases. The detectors charge quickly on output signal and dischargeat the selected time-constant rate, as shown in FIG. 7. l Referring again to FIG. 3, the upper limit of comparator 56 of conventional design is set to trigger on amplitudes above the ECG modulated subcarrier amplitudes. These higher amplitude signals are caused by noise received by the antenna when the transmitter is out of range or no longer transmitting. The latter occurs when the low battery circuit in the transmitter shuts down transmission. When the output of the longer time constant detector 52 reaches a voltage higher than a reference voltage that corresponds to the uppertrigger amplitude, the comparator energizes the range/battery inoperative indicator 4 8.

The lower limit of the comparator 56' is set to trigger on amplitudes below the ECG modulated subcarrier amplitude. These lower amplitude signals result from an unmodulated or off channel interfering frequency. When the output of the shorter time-constant detector 54 decreases to a voltage below a reference voltage that corresponds to the lower trigger voltage, the comparator 56 energizes the range/battery inoperative indicator 48. Thus the window comparator 56 is unresponsive to signals within its window, but for signals above or below set limits, it energizes the range/battery inoperative circuitry.

Besides energizing the inoperative indicator 48, the comparator triggers the output hold off circuitry 41 when it detects an inoperative condition. The hold off grounds the output terminal 40 to prevent an erroneous output from the receiver. The comparator also disables the electrode inoperative circuitry 42, 44, 50 to prevent an erroneous indication of a detached electrode when there is a range/battery inoperative condition.

The window comparator is connected to the local oscillator loop in the demodulation circuitry. The comparator output controls the automatic frequency control (AFC) 30 to regulate the local oscillator frequency. When the comparator detects an inoperative condition, it energizes the free running multivibrator 60. The multivibrator varies the local oscillator frequency from near one band edge and then releases it to the control of the AFC loop. If the inoperative circuit does not clear, the multivibrator then sets the local oscillator to a frequency near the other band edge and releases it to the control of the AFC loop. This action continues until the receiver locks on an appropriate re- -ceived signal. When a transmitted signal is received and detected, the range/battery inoperative circuitry 41, 60, 61, 62 and the indicator. 48 will turn off. Then the receiver will operate normally.

As shown, this invention detects and indicates certain malfunctions of an ECG telemetry system. Accurate detection and prompt indication of malfunctions are invaluable to ECG monitoring systems because they permit continual monitoring of the patients actual heart condition. Without them, it would be more difficult for the system operator to determine the cause of an unusual ECG signal.

We claim:

1. A system for wireless transmission of electrical signals, comprising:

an input circuit having an electrode for receiving an input signal applied thereto;

transmitter means including frequency-control means connected to said input circuit for radiating an output signal that is modulated by a subcarrier signal means which is modulated within a preselected band of frequencies by an input signal applied to said electrode, and means for radiating said output signal modulated by a said subcarrier signal having a signal frequency outside the limits of said band in response to an open-circuit electrode of the input circuit; reception means including a demodulator adapted to receive the radiated. output signal for producing said subcarrier signal therefrom; and detector means connected to said reception means for producing a first output indicative of reception of the radiated output signal modulated by said signal frequency.

2. A system as in claim 1 wherein:

said reception means includes first and second detectors, each connected to receive the demodulated subcarrier signal, the first detector producing therefrom a first signal representative of the peak amplitudes of said subcarrier signal over a given first time interval and said second detector producing therefrom a second signal representative of the peak amplitudes of said subcarrier signal over a given second time interval which is shorter than said first time interval; and comprising comparator means operable for comparing applied signals with a pair of signal levels, said comparator means being connected to said first and second detectors for producing a second output indicative of an inoperative condition in response to the first signal from the first detector attaining a value greater than a first one of said pair of signal levels and the second signal from the second detector attaining a value less than a second one of said pair of signal levels.

3. A system as in claim 2 wherein:

the transmitter means includes an FM-FM radio wave transmitter having an electrode of said input circuit adapted to be attached to the body of a patient for receiving an input signal therefrom;

said frequency-control means sets the signal frequency of said subcarrier signal to a frequency lower than the frequencies of said band in response to an electrode detaching from the body of a patient; and

logic means connected to said comparator means and to said detector means for inhibitingthe first output indicative of reception of the radiatedoutput signal modulated by said signal frequency in. re-

sponse to said second output produced bysaid" modulating a detected input signal to produce a subcarrier signal;

second means connected to said first means for demodulating the subcarrier signal to produce an information signal;

output means connected to said second means for deriving the information signal from the receiver;

a first detector connected to said first means for converting the peak amplitude of the subcarrier signal over a given first time interval to a representative first DC signal;

a second detector connected to said first means that converts the peak amplitude of the subcarrier signal over a given second time interval to a representative second DC. signal, said first time interval being longer than said second time interval; and

comparator means operable for comparing applied signals with a pair of signal levels;

said comparator means being connected to said first and second detectors for producing an output indication of an inoperative condition in response to the first DC. signal of the first detector attaining a value greater than .a first one of. said pairofsignal levelsor in response to the second DC. signal of the second detector-attaining a value less than a second one of said pair of signal levels.

' 6. A receiver asin claim 5 wherein said first means includes a demodulator and a local oscillator signal generator connected thereto; and comprises circuit means connected to said comparator means and tosaid signal generator for varying the frequency of the signal produced thereby in response to the output indication of an inoperative condition producedby said comparator means.

7. A receiver as in claim 6 comprising logic means connected to said output means and to said comparator means for inhibiting the information signal from the output means in response to the output indication of an inoperative condition produced by said comparator means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. ,768,01 Dat d October 23, 1973 Inven r.( Richard F. Dillman et al.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the drawings, Sheet 1]., Fig. 3, insert a connection line between the bottom of block 61 and the output of comparator 56.

Signed and sealed this 8th day of April 1975.

(SEAL Attest:

C. MARSHALL DANN RUTH C. I'EASOTE Commissioner of Patents Attesting Gfficer and Trademarks FORM P0-1050 (10-69) uscoMM-oc 60376-F'69 ".5. GOVERNMENT PRINTING OFFICE "II O-Jli-J! I ITED STAT-ES, O E F ATE o Fj CORRECTION Patent 'N 3.768.017 Dated QctOQe'r 23. 1973 Inventor(s) Richard F. Dillman, et a1.

pears in the above-identified patent It is certified that error ep d as shownbelow:

and'that said Letters Patent are hereby correcte Column: 6, line 16, "30" should read 39 Signed vand sealed this 1st day of October 1974,

' (SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-1050 (10-69) uscoMM-Dc eoa'le-pos U.5. GOVERNMENT HUNTING OFFICE HID 0-366-334 

1. A system for wireless transmission of electrical signals, comprising: an input circuit having an electrode for receiving an input signal applied thereto; transmitter means including frequency-control means connected to said input circuit for radiating an output signal that is modulated by a subcarrier signal means which is modulated within a preselected band of frequencies by an input signal applied to said electrode, and means for radiating said output signal modulated by a said subcarrier signal having a signal frequency outside the limits of said band in response to an open-circuit electrode of the input circuit; reception means including a demodulator adapted to receive the radiated output signal for producing said subcarrier signal therefrom; and detector means connected to said reception means for producing a first output indicative of reception of the radiated output signal modulated by said signal frequency.
 2. A system as in claim 1 wherein: said reception means includes first and second detectors, each connected to receive the demodulated subcarrier signal, the first detector producing therefrom a first signal representative of the peak amplitudes of said subcarrier signal over a given first time interval and said second detector producing therefrom a second signal representative of the peak amplitudes of said subcarrier signal over a given second time interval which is shorter than said first time interval; and comprising comparator means operable for comparing applied signals with a pair of signal levels, said comparator means being connected to said first and second detectors for producing a second output indicative of an inoperative condition in response to the first signal from the first detector attaining a value greater than a first one of said pair of signal levels and the second signal from the second detector attaining a value less than a second one of said pair of signal levels.
 3. A system as in claim 2 wherein: the transmitter means includes an FM-FM radio wave transmitter having an electrode of said input circuit adapted to be attached to the body of a patient for receiving an input signal therefrom; said frequency-control means sets the signal frequency of said subcarrier signal to a frequency lower than the frequencies of said band in response to an electrode detaching from the body of a patient; and logic means connected to said comparator means and to said detector means for inhibiting the first output indicative of reception of the radiated output signal modulated by said signal frequency in response to said second output produced by said comparator means.
 4. A system as in claim 1 wherein said reception means also produces an information signal from said subcarrier signal which is representative of said input signal; and comprising inhibit means connected to said reception means for inhibiting said information signal from the reception means in response to said first output from said detector means.
 5. A receiver comprising: reception means for detecting radiated electrical signals; first means connected to said reception means for demodulating a detected input signaL to produce a subcarrier signal; second means connected to said first means for demodulating the subcarrier signal to produce an information signal; output means connected to said second means for deriving the information signal from the receiver; a first detector connected to said first means for converting the peak amplitude of the subcarrier signal over a given first time interval to a representative first D.C. signal; a second detector connected to said first means that converts the peak amplitude of the subcarrier signal over a given second time interval to a representative second D.C. signal, said first time interval being longer than said second time interval; and comparator means operable for comparing applied signals with a pair of signal levels; said comparator means being connected to said first and second detectors for producing an output indication of an inoperative condition in response to the first D.C. signal of the first detector attaining a value greater than a first one of said pair of signal levels or in response to the second D.C. signal of the second detector attaining a value less than a second one of said pair of signal levels.
 6. A receiver as in claim 5 wherein said first means includes a demodulator and a local oscillator signal generator connected thereto; and comprises circuit means connected to said comparator means and to said signal generator for varying the frequency of the signal produced thereby in response to the output indication of an inoperative condition produced by said comparator means.
 7. A receiver as in claim 6 comprising logic means connected to said output means and to said comparator means for inhibiting the information signal from the output means in response to the output indication of an inoperative condition produced by said comparator means. 